This invention relates to a free machining, non-magnetic, austenitic stainless steel alloy and a magnetically biased device such as an accelerometer made therefrom. More particularly, the alloy provided has improved machinability, improved freedom from ferrite and a consistent match in its coefficient of thermal expansion (COE) with that required for coacting with a magnetic component of the magnetically biased device over a given temperature range.
The present invention stems from the discovery that difficulties hitherto encountered in the production and operation of magnetically biased accelerometers such as, for example, described in U.S. Pat. No. 4,329,549 issued on May 11, 1982 to D. S. Breed, have been caused primarily by certain less than desirable characteristics of the alloys used to fabricate the non-magnetic component of the device. In order for a magnetically biased accelerometer to function properly, this component must maintain the desired relationship with the magnetic component to close tolerances over a desired operating temperature range. However, the alloys used to fabricate the non-magnetic component exhibit excessive variations in dimensional tolerances because of less than desired machinability, small but excessive variations in the required COE and, because of the presence of greater than tolerable amounts of ferrite, more than the desired magnetic permeability. Such variations have resulted in an unacceptable rate of rejection of finished devices. These vexing problems have been encountered even though the non-magnetic austenitic stainless steel member which coacts with the magnetic member in such devices has been made of AISI Type 309S having the following composition in weight percent:
______________________________________ w/o ______________________________________ Carbon 0.8 max. Manganese 2.00 max. Silicon 1.00 max. Phosphorus 0.045 max. Sulfur 0.030 max. Chromium 22-24 Nickel 12-15 Iron Bal. ______________________________________